1. Field of the Invention
The present invention relates to electrostatic spring softening applied to mechanical resonators, and in particular to improved microgyroscope resonators and their manufacture. More particularly, this invention relates to producing and tuning microgyroscopes, and particularly, isolated resonator gyroscopes.
2. Description of the Related Art
Gyroscopes are used to determine direction based upon the sensed inertial reaction of a moving mass. In various forms they are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to autonomously determine the orientation of a free object. A number of recent technologies have brought new forms of gyroscopes, including optical gyroscopes such as laser gyroscopes and fiberoptic gyroscopes as well as vibratory gyroscopes. Such vibratory gyroscopes generally operate using electrostatic actuation and sensing, employing controlled capacitance gaps between a moving resonator and a supporting structure, e.g. a baseplate or package. These and other new gyroscopes greatly widen the range of gyroscope applications because they offer increasing performance levels at lower costs.
Some prior vibratory gyroscopes with symmetric designs have been produced, however their vibratory momentum is transferred directly to their baseplates or packages. This coupling admits external disturbances and energy loss indistinguishable from inertial rate input and hence leads to sensing errors and drift. Other planar tuning fork gyroscopes may achieve a degree of isolation of the vibration from the baseplate, however these gyroscopes lack the vibrational symmetry desirable for tuned operation. One example of such a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090 to Tang et al. which describes a symmetric cloverleaf vibratory gyroscope design and is hereby incorporated by reference herein.
Recently, a variety of new resonator gyroscopes have been developed which include a resonator that is isolated from the baseplate or supporting structure. Some of these vibratory gyroscopes include various post resonator gyroscopes such as described in U.S. application 09/928,279, 10/370,953, 10/410,744, by A. Dorian Challoner et al., filed Apr. 10, 2003, and entitled xe2x80x9cISOLATED RESONATOR GYROSCOPE WITH COMPACT FLEXURESxe2x80x9d and Ser. No. 10/423,459 by A. Dorian Challoner et al., filed on Apr. 25, 2003, and entitled xe2x80x9cISOLATED RESONATOR GYROSCOPE WITH ISOLATION TRIMMING USING A SECONDARY ELEMENTxe2x80x9d.
Vibration isolation using a low-frequency seismic support is also known (e.g., U.S. Pat. No. 6,009,751, which is incorporated by reference herein), however such increased isolation comes at the expense of proportionately heavier seismic mass and/or lower support frequency. Both effects are undesirable for compact tactical inertial measurement unit (IMU) applications.
In the case of many of the recent resonator gyroscope which use electrostatic excitation and sensing such as some of those mentioned above, mechanical tuning of the gyroscope modes is often necessary to obtain optimum performance, e.g. navigation grade performance. Mechanical tuning, e.g. laser ablation or focused ion beam machining, of such resonator gyroscopes is very expensive to perform after the manufacturing process. In addition, it is difficult to correct for perturbations which will result from the gyroscope packaging as the mechanical tuning operations are performed prior to final packaging. Such mechanical tuning cannot be used to recalibrate the gyroscope in the field after final packaging.
There is a need in the art for small gyroscopes with greatly improved performance for navigation and spacecraft payload pointing. Further, there is a need for such gyroscopes to be easily tunable and capable of selective control of differential rocking modes and balance control. Still further, there is a need for techniques which allow tuning of such gyroscopes without machining. Finally, there is also a need for gyroscopes and tuning techniques that allow for recalibration in the field after final packaging. These and other needs are met by the present invention.
The present invention discloses the use of electrostatic spring softening to induce a balanced condition while simultaneously minimizing frequency split. This invention has application to various of vibratory Coriolis force sensing gyroscopes using electrostatic driving and sensing, but particularly those with isolated or balanced resonators. For example, embodiments of the invention can be employed in various isolated resonator gyroscopes such as described in U.S. application Ser. Nos. 09/928,279, 10/370,953, 10/410,744 by A. Dorian Challoner et al., filed Apr. 10, 2003, and entitled xe2x80x9cISOLATED RESONATOR GYROSCOPE WITH COMPACT FLEXURESxe2x80x9d and Ser. No. 10/423,459 by A. Dorian Challoner et al., filed Apr. 25, 2003, and entitled xe2x80x9cISOLATED RESONATOR GYROSCOPE WITH ISOLATION TRIMMING USING A SECONDARY ELEMENTxe2x80x9d. More generally, the invention can be used in any redundant degree of freedom mechanical resonator in any application which may require one. Typical embodiments of the invention include a balanced redundant degrees-of-freedom resonator structure combined with electrostatic spring softening to tune both mechanical modes (and axes) and mechanical quality (Q) factor (and dampening axes).
One significant advantage of the present invention is that it allows post-manufacture tuning using simple electronics on a fully vacuum packaged gyroscope and can be performed in the field as a calibration routine to compensate for aging, radiation damage and other effects which result in a gradual decay in performance. Thus, the invention can yield an affordable vibratory gyroscope with navigation grade performance by means of a precision isolated symmetric planar resonator of optimum scale that can be fabricated with silicon photolithography from commercial double-side polished silicon wafers with low total thickness variation.
In one exemplary embodiment, the present invention can be employed with an isolated resonator comprising two bodies with transverse inertia symmetry about an axis aligned with an input axis and elastically supported so that their axes of symmetry and centers of mass coincide and together form two differential rocking modes of vibration transverse to the axis of symmetry. The two bodies are supported on a baseplate having an inertial rate input axis and exhibit substantially equal frequencies distinct from other modes of vibration, mutually orthogonal and imparting substantially zero net momentum to the baseplate. Excitation and sense electrodes are disposed below the resonator on the supporting baseplate structure to excite and sense movement of the resonator.
In the exemplary embodiments which follow, a first one of the bodies is a proof mass, a second one of the bodies is a counterbalancing plate. The counterbalancing plate is for reacting with the excitation and sense electrodes. However, other structures and arrangements which can employ the inventive principle of electrostatic spring softening will be apparent to those skilled in the art.
In general, the excitation and sense electrodes are disposed below the counterbalancing plate. The excitation electrodes are aligned to drive a first one of the differential rocking modes to vibrate. The sense electrodes are aligned to sense the motion of the second differential rocking mode induced by Coriolis accelerations resulting from the inertial rate input and internally driven differential rocking motion about the first mode axis.
A key element of the present invention is the incorporation of a plurality of bias electrodes which are used to tune isolation of the resonator from the baseplate and minimize frequency split between the excitation and sensing vibration modes. Typically, the bias electrodes are disposed on the baseplate beneath the two bodies of the resonator to provide a bias against it. The addition of these bias electrodes permits selective control of the differential rocking mode and electrostatic balance control. In addition to isolated resonator gyros, the secondary element is also applicable to other types of vibratory gyroscopes, such as hemispherical resonator gyroscopes.
The present invention discloses an isolated electrostatic biased resonator gyroscope. The gyroscope includes an isolated resonator having a first and a second differential vibration mode, a baseplate supporting the isolated resonator, a plurality of excitation affixed to the baseplate for exciting the first differential vibration mode, a plurality of sensing electrodes affixed to the baseplate for sensing movement of the gyroscope through the second differential vibration mode and a plurality of bias electrodes affixed to the baseplate for trimming isolation of the resonator and substantially minimizing frequency split between the first and second differential vibration modes. Typically, the isolated resonator comprises a proof mass and a counterbalancing plate joined and supported by flexures with the bias electrodes disposed on the baseplate below.